Styrene (ST) is an important monomer for producing polystyrene (PS), ABS resin, styrene-butadiene rubber, etc., and it is mainly produced by a process of dehydrogenating ethylbenzene. In recent years, along with the development and scale-up of ethylene industry, the technology of recovering styrene from pyrolysis gasoline draws more and more attention.
Pyrolysis gasoline is a by-product of ethylene industry, of which output is about 60 to 70% of ethylene output. C8 fraction of pyrolysis gasoline is rich in styrene and mixed xylenes. An ethylene plant in 1000 kt/a scale can produce 24 to 42 kt/a of styrene, and at the same time mixed xylenes can be recovered. The production cost of styrene recovered from pyrolysis gasoline is about ½ of that of styrene produced by a process of dehydrogenating ethylbenzene.
The process for recovering styrene from pyrolysis gasoline that is widely regarded as being feasible at present is an extraction-distillation process. However, phenylacetylene (PA) and styrene are similar in chemical structure, and they also have similar interaction with an extraction-distillation solvent, so that it is impossible to achieve an effective separation of styrene from PA by the extraction-distillation. The presence of PA will not only increase the consumption of catalyst during anionic polymerization of styrene and affect chain length and polymerization rate, but also lead to worsen properties of polystyrene, e.g., off-color, degradation, odor-releasing and the like. Therefore, it is necessary to remove phenylacetylene from a styrene stream, while the loss of styrene should be as low as possible. Accordingly, the development of a high selectivity catalyst for the selective hydrogenation of phenylacetylene and of a relevant process becomes a key of the technology of recovering styrene from pyrolysis gasoline.
Patent application CN1852877A discloses a process for the reduction of phenylacetylene impurity in the presence of styrene monomer. A styrene monomer stream containing a minor amount of phenylacetylene is supplied to a hydrogenation reactor, and a hydrogenation gas comprising hydrogen is also supplied to the hydrogenation reactor. The styrene monomer stream and the hydrogen are brought into contact with a catalyst bed containing a catalyst comprising a reduced copper compound on a θ-alumina support. The hydrogenation reactor is operated at a temperature of at least 60° C. and a pressure of at least 30 psig to hydrogenate phenylacetylene to styrene. The hydrogenation gas comprises a mixture of nitrogen and hydrogen. This technique is featured with a relatively high reaction temperature, a low hydrogenation rate of phenylacetylene (about 70%), a short lifetime of catalyst, and a high loss of styrene (about 3%).
Patent application CN1087892A discloses a process and apparatus for purifying styrene monomer in a styrene stream by hydrogenation, wherein a diluent such as nitrogen is used to dilute hydrogen, the hydrogen is supplied by a vent gas from the dehydrogenation of ethylbenzene, and phenylacetylene impurity is hydrogenated to styrene by the aid of a multi-stage catalyst bed reactor. This patent is only directed to a process for the selective removal of phenylacetylene from a styrene stream containing phenylacetylene at a low concentration such as 300 ppm. On the other hand, the catalyst used exhibits a low hydrogenation rate of phenylacetylene (about 95%), and the loss of styrene is about 0.2%.
Thus, there is still need for a process to selectively hydrogenate phenylacetylene with a high selectivity useful in the technology for recovering styrene from pyrolysis gasoline.